The present invention pertains to a bandpass filter for use in the super-high-frequency (SHF) band, and particularly to a bandpass filter having parallel-coupled lines which uses microstrip lines as a resonator.
Generally, a bandpass filter for use in the SHF band is employed for the output port of an SHF transmitter, the input port of an SHF receiver and the output port of a frequency converter, so as to reduce the insertion loss of a transmitted signal and to enhance the capability of removing unwanted frequencies. Such a bandpass filter is utilized in an amplifier and frequency converter required for the configuration of ground microwave systems and satellite communication systems. SHF bandpass filters have been recently constructed such that an array of microstrip lines are formed in parallel. However, in the bandpass filter having parallel-coupled lines and using microstrip lines, the distance between parallel-coupled lines of the first and last parallel microstrip lines is below a specific value (0.1 mm), which makes the manufacturing of the filter difficult. Therefore, during filter design, the precise estimation of the insertion loss and bandwidth of such a bandpass filter is difficult.
Such problems will be described below in detail with reference to the attached drawings.
FIG. 1 is a schematic view of a general four-terminal parallel-coupled transmission line.
Referring to FIG. 1, the parallel-coupled transmission line comprises terminals 1 and 4 which constitute an input port, terminals 2 and 3 which constitute an output port, and microstrip lines 5 and 6 disposed in parallel while being spaced apart by a distance d and each characterized by having a length l and a width W. Here, length l has a value corresponding to one fourth the wavelength (.lambda./4) of a signal.
FIG. 2 is a schematic view of a conventional bandpass filter having parallel-coupled lines and using a stepped impedance resonator. Referring to FIG. 2, two-terminal parallel-coupled lines BL.sub.1 .about.BL.sub.n+1 (wherein terminals 3 and 4 of the four-terminal parallel-coupled line of FIG. 1 are left open) are consecutively arranged in a step form. The two-terminal parallel-coupled lines BL.sub.1 .about.BL.sub.n+1 are formed with microstrip lines SL.sub.1 .about.SL.sub.2n+2 which are disposed so as to have different distances d.sub.1 .about.d.sub.n+1. Impedance Z.sub.0 indicates the characteristic impedance of the input line and output line.
FIG. 3A is an equivalent circuit diagram of an arbitrary (i+1)th two-terminal parallel-coupled line BL.sub.i+1 of the bandpass filter having parallel-coupled lines shown in FIG. 2. Referring to FIG. 3A, for admittance inverter j.sub.(i,i+1), the characteristic impedance thereof equals that of the input/output lines of the bandpass filter. Also, input/output lines .phi.L.sub.2i and .phi.L.sub.2i+1 are each one quarter wave in length.
FIG. 3B is an equivalent circuit diagram of the bandpass filter having parallel-coupled lines shown in FIG. 2. Referring to FIG. 3B, n+1 admittance inverters j.sub.(0,1) .about.j.sub.(n,n+1) are connected in series via input/output lines .phi.L.sub.0 .about..phi.L.sub.2n+1 each of which are also one quarter wave in length. The characteristic impedance of the quarter-wavelength input/output lines .phi.L.sub.0 .about..phi.L.sub.2n+1 is equal to input/output impedance Z.sub.0 of the bandpass filter. Therefore, the impedances Z(e).sub.0 (even mode) and Z(o).sub.0 (odd mode) of each of the two-terminal parallel-coupled lines BL.sub.1 .about.BL.sub.n+1 shown in FIG. 2 are expressed as follows: EQU Z(e).sub.0(i,i+1) =Z.sub.0 [1+Z.sub.0 .multidot.j.sub.(i,i+1) {Z.sub.0 .multidot.j.sub.(i,i+1) }.sup.2 ] (1) EQU Z(o).sub.0(i,i+1) =Z.sub.0 [1+Z.sub.0 .multidot.j.sub.(i,i+1) -{Z.sub.0 .multidot.j.sub.(i,i+1) }.sup.2 ] (2)
Using Equations (1) and (2), if the impedances of the even mode and odd mode of the first and last parallel-coupled lines BL.sub.1 and BL.sub.n+1 of the bandpass filter shown in FIG. 2 are calculated, it is noted that the impedances Z(e).sub.0(0,1) and Z(o).sub.0(0,1) of the first parallel-coupled line is the same as the impedances Z(e).sub.0(n,n+1) and Z(O).sub.0(n,n+1) of the last parallel-coupled line. Using the impedance values of the even mode and odd mode, if the width W of microstrip lines SL.sub.1, SL.sub.2, SL.sub.2n+1 and SL.sub.2n+2 constituting the first and last parallel-coupled lines BL.sub.1 and BL.sub.n+1 and distance d between the microstrip lines are calculated, the value of distance d is less than 0.1 mm. This is not easy to accomplish with ordinary print circuit boards (for instance, epoxy-glass boards).
To overcome the above problem (when d&lt;0.1 mm), Mitsuo Makimoto and Sadahiko Yamashita (both of Japan) have disclosed in a paper entitled "Strip-line Resonator Filters having Multi-coupled Sections (IEEE MTT-S DIGEST, pp. 92-94, 1983), that the first and last terminal pairs BL.sub.1 and BL.sub.n+1 of the bandpass filter shown in FIG. 2 are multi-coupled, as shown in FIG. 4. However, in a filter having such a structure, the microstrip lines are discontinuous in the multi-coupled portions 30 and 31, which causes errors in circuit interpretation and thus impedes the precise estimation of insertion loss and bandwidth of a specific bandpass filter during design.